You will love the RSVP. The scenery is beautiful and most of the hills are rollers. Far less crowded than STP and atmosphere of the ride is much more relaxed! As long as you do some hill training and take it at your own pace you will do fine. Party at the end is a blast. Folks were taking off their shoes and socks and soaking in the hotel fountain (The gleam of sweat and sunscreen in the water was kind of gross)!
Chuckanut drive is so pretty you won't even think about the hills, you will be too busy taking in the view. We just rode it about a month ago in the Chuckanut Century (both ways) and it was the best part of the fide for sure.
The only part that is a bummer is the border crossing on the way back on the bus. The wait was unbelievable.